Die-casting prevails widely as a technology for casting a product at a high speed with a high degree of accuracy. Then plural products are assorted in accordance with mold locking force by a die-casting machine maker.
When you buy a new die-casting machine or when you cast a new product with an already bought die-casting machine, you need to judge whether or not it is possible to cast a die-casting product of prescribed quality. Here, a die-cast quality is judged by nests, fins, cold folds, and cold shut for example. In general, a larger machine is more likely to satisfy a prescribed quality but the price increases. Hence it is important to select a machine that satisfies a prescribed quality and is less expensive.
Whether or not a prescribed quality is satisfied is judged through the following procedure. Firstly, the conditions of a die-casting machine, a die assembly, and molten metal are set. The conditions are collectively called die-casting conditions (casting methods). Successively, under the die-casting conditions, a value of a parameter used for quality judgment (called a “quality judging parameter”) is compared with a judging standard value functioning as a standard for judging whether or not a prescribed quality is satisfied and thereby whether or not the prescribed quality is satisfied is judged. The procedure is hereunder explained more specifically.
FIG. 11 is a table showing quality judging parameters, inequalities to restrict the quality judging parameters, and prescribed qualities satisfied when the inequalities are true. The inequalities are formed so that the quality judging parameters (the left-hand sides of the inequalities) may be restricted by the judging standard values (the right-hand sides of the inequalities) functioning as the standards for judging whether or not the prescribed qualities are satisfied. Then, when the inequalities are true, the prescribed qualities are satisfied. FIG. 12 is a table showing the expressions to obtain the judging standard values. Here, the meanings of the symbols are shown in FIG. 14. Then the symbols are known widely and thus the explanations thereof are omitted.
Explanations are made on the basis of the case where a filling time (Tf) is adopted as an example of the quality judging parameters. In this case, the judging standard value is TfUB (the upper limit of the filling time). When the inequality (Tf≦TfUB) to restrict the quality judging parameter (Tf) by the judging standard value (TfUB) is true, it is judged that the prescribed quality of “few misruns” is satisfied. Here, the judging standard value TfUB is obtained from the expression TfUB=Kf×Rm2 as shown in the table of FIG. 12. Here, Kf and Rm represent a material coefficient and a product thickness respectively (refer to FIG. 14).
The selection of a die-casting machine is carried out by judging the qualities of plural quality judging parameters as stated above and evaluating whether or not a cast die-casting product satisfies the plural prescribed qualities. When the selection of a die-casting machine is carried out and even one of the quality judging parameters not satisfying a prescribed quality exists, it is judged that it is impossible to cast a product of the prescribed qualities. On this occasion, the die-casting conditions are changed and the selection of a die-casting machine is carried out again. The change of the die-casting conditions and the selection of a die-casting machine are repeated until all the quality judging parameters satisfy all the prescribed qualities and finally die-casting conditions satisfying all the prescribed qualities are obtained.
Meanwhile, in quality judgment, it is sometimes necessary to use a value of molten metal filling pressure and a square value of a molten metal flow rate at actual casting. On this occasion, a P-Q2 diagram (refer to Non-patent Documents 1 and 2) is used in many cases. The P-Q2 diagram is explained hereunder.
FIG. 13 is a graph showing an example of the P-Q2 diagram. In the P-Q2 diagram (1), a machine characteristic line 11 and a die characteristic line 12 are drawn on a two-dimensional coordinate plane expressing a molten metal filling pressure (P) on the vertical axis 51 and a square of a molten metal flow rate (Q2) on the horizontal axis 52. The machine characteristic line 11 is a straight line showing the casting capability of a die-casting machine used and is represented by the expression P=PO×(1−Q2/QO2). Here, PO and QO are represented by the expressions PO=Pacc×Ac/As and QO=VsO×As respectively and Pacc represents an accumulator pressure, VsO an injection velocity at blanking, Ac an injection cylinder cross section, and As a sleeve cross section. The die characteristic line 12 is a characteristic line intrinsic to a die assembly and is represented by the expression P=B×Q2. Here, B is represented by the expression B=Dm/(2×Ag2×Cg2) and Dm represents a molten metal density, Ag a gate cross section, and Cg a flow rate coefficient.
In the P-Q2 diagram (1), the intersection 13 of the machine characteristic line 11 and the die characteristic line 12 is called a “process point”. The coordinate value Q12 of the process point 13 on the horizontal axis is the square value of a molten metal flow rate at actual casting and the coordinate value P1 on the vertical axis is a molten metal filling pressure at the actual casting.
Non-patent Document 3 discloses a method for determining die-casting conditions by judging whether or not a process point exists within a process window in a P-Q2 diagram and judging whether or not a J-Factor is in a prescribed range. It is possible to determine die-casting conditions by repeating the selection of a die-casting machine and the change of die-casting conditions until prescribed qualities are satisfied.    Non-patent Document 1: “Encyclopedia of Die-casting” supervised by 50th Anniversary Editorial Committee of Japan Diecasting Association, published by Keikinzoku Tsushin AL Co., Ltd., 2005, p. 356-357    Non-patent Document 2: “Aluminum Alloy Die-Casting—The Technology and Troubleshooting” authored by Tomonobu Sugano and Torazou Uehara, published by Kallos Publishing Co., Ltd., 1988, P. 138-143    Non-patent Document 3: “Application of PQ2 Diagram and J-Factor to Evaluate Parameters for High-Pressure Die Casting Process” authored by Yoshio Kaneuchi, Hitachi Metals Technical Review, published by Hitachi Metals, Ltd., 2007, Vol. 23, p. 27-32